The present invention relates to an optical amplifier for optical communications, and more particularly, to a multi-stage fiber amplifier in which the total available pump power, which is supplied from a single source, is optimally divided between the stages to provide maximum gain and minimum noise for a given spectral response.
High output power amplifiers are required for many applications, including, for example, multi-wavelength systems. One of the key performance characteristics of an amplifier is its output power, which is largely governed by how efficiently the pump light is converted into signal light.
FIG. 1 schematically illustrates that at the input of gain fiber 10 both the signal and the spontaneous emission (SE) are available to be amplified. The amount of SE that is amplified must be reduced relative to the amount of signal that is amplified so that signal amplification dominates in the utilization of pump power. The size of the signal is typically set by the system link loss and therefore cannot be increased. The amount of amplified spontaneous emission (ASE) could be reduced by reducing the total pump power. However, there would then be less pump power available for conversion to signal power. Since the erbium-doped fiber amplifier has utility in communication systems operating at 1550 nm, that fiber amplifier is specifically discussed herein by way of example. The invention also applies to fiber amplifiers containing gain ions other than erbium, since ASE also diverts pump power from the signal in amplifiers utilizing gain ions other than erbium. As shown by curve 18 of FIG. 2, the gain spectrum of a highly inverted erbium-aluminum-doped germania silicate fiber amplifier has a peak around 1532 nm and a broad band with reduced gain to about 1560 nm. Some prior art fiber amplifiers included means for reducing the 1532 nm peak to prevent the occurrence of such disadvantageous operation as wavelength dependent gain or gain (with concomitant noise) at unwanted wavelengths. The resultant gain spectrum might be as represented by curve 19, for example.
In one such fiber amplifier, a filter such as a fiber containing ions that absorb at the ASE wavelength is connected between two sections of gain fiber. The ASE is filtered from the output of the first gain fiber section by the ASE-absorbing fiber, and the resultant signal light is amplified by the second gain fiber section. Both the signal and pump light are introduced into the first gain fiber section, and the remnant pump light at the output of the first gain fiber section is used to pump the second gain fiber section. The lengths of gain fiber at both ends of the filter determine the amplifier gain and spectral response.
A modification of the above-described amplifier is described in the publication, R. I. Laming et al. "High-Sensitivity Two-Stage Erbium-Doped Fiber Preamplifier at 10 Gb/s", IEEE Photonics Technology Letters, vol. 4, No. 12, December 1992, pp. 1348-1340. In the fiber amplifier disclosed in that publication, the ASE absorbing fiber of the above-described amplifier is replaced by a 1536 nm isolator for suppressing the backward-traveling ASE. Since the isolator has a very high loss at the 980 nm pump wavelength, two wavelength division multiplexer (WDM) couplers are included in the circuit to provide a low-loss pump power bypass around the isolator.
Another fiber amplifier having ASE filtering is described in the publication "High Gain Two-Stage Amplification with Erbium-Doped Fibre Amplifier" by H. Masuda et al., Electronics Letters, 10 May 1990, vol. 26, No. 10, pp. 661-662. The ASE filter is connected between two fiber amplifier stages. Separate pump sources are connected to the two stages, the second stage being double pumped from two separate sources, whereby it receives more pump power than the first stage. The signal is amplified by the first fiber amplifier stage. The ASE wavelengths in the output of that amplifier are filtered, and the resultant signal is connected to the second amplifier stage. Since the input signal is relatively small, much of the pump power will be converted to ASE in the first stage. With the ASE filter in place, this ASE optical power is lost, as it does not reach the second stage. Therefore, in this type of configuration, the amount of power converted to ASE is minimized by reducing the ASE build up in the first stage by limiting the amount of pump power supplied to the first stage. In this kind of fiber amplifier, the pump conversion is enhanced, but a large number of pump sources is required.
U.S. Pat. No. 5,050,949 also teaches a two-stage fiber amplifier in which unequal pump power is supplied to each of the two stages. The gain spectrums of the two gain fibers are made different by selecting gain fibers having different host glasses and/or different gain ions. Since the gain spectrums of the two stages are different, signal gain equalization is achieved, rather than optimization of pump power conversion.